1. Field of the Invention
The present invention relates to a metal halogen electrodeless illumination lamp, and particularly, to a gas discharging lamp in use for illumination instruments especially, to lamps containing a fill substance which, once excited with a discharge, emits optical radiation in the visible part of the spectrum, and more precisely to gas discharge illuminating lamps in which a metal halogen or a mixture of metal halogens is the primary component of the fill substance, and which can be used in various illumination applications such as illumination of streets and large premises
2. Description of the Background Art
Metal halogen illumination lamps have been well known since the mid-1960""s and have found extensive application owing to the high intensity of generated radiation. A metal halogen illumination lamp generally comprises a quartz tube filled with a mixture of substances capable of sustaining an arc discharge, and a glass envelope enclosing said quartz tube.
The quartz tube contains two electrodes between which an arc discharge is started when the lamp is operated. The primary component in the quartz tube fill is mercury. Additionally, a fill substance includes an inert gas which aids in starting a discharge, and one or a mixture of metal halogens, mostly iodides (refer to U.S. Pat. No. 3,234,421). The desired spectral output is provided by using halides of various metals and their compounds. For example, BiI3 (U.S. Pat. No. 3,989,972) compounds of Sn (U.S. Pat. No. 4,001,626), halides of sodium lithium and scandium (U.S. Pat. No. 4,247,798), Ti halide (U.S. Pat. No. 4,866,342), or Na and Ti iodides in a certain proportion (U.S. Pat. No. 5,225,738).
The major drawback of metal halogen lamps of this type is that they use mercury as the quartz tube fill substance, which generally amounts to tens of milligrams.
The hazards involved in manufacturing and utilization of mercury containing lamps have been recognized in that mercury is a highly toxic substance. Besides, there is a considerable loss of heat in these lamps to electrodes contained inside a quartz tube, and the electrodes material vaporizes, darkening the quartz tube, which limits the performance potential of the lamp.
Further research into ways to ensure a desired performance characteristics of mercury containing metal halogen illumination lamps has led to development of gas discharge electrodeless illumination lamps. These consist of an electromagnetic radiation source, for example, a microwave generator coupled via a coupling means with a microwave cavity containing a discharge bulb. The discharge bulb contains a fill substance (or a mixture of substances) which, when excited with a microwave discharge occurring therein, emits optical radiation that features a molecular spectrum. A certain part of the microwave cavity walls serves as a microwave screen which generally is a metal mesh constructed so that it is opaque to microwave energy, while being practically transparent to optical radiation.
This type lamps is known as gas discharge electrodeless illumination lamps as disclosed in U.S. Pat. No. 5,404,076, U.S. Pat. No. 5,661,365, U.S. Pat. No. 5,798,611 U.S. Pat. No. 5,834,895, U.S. Pat. No. 5,866,980. As the primary components in the discharge bulb fill substance these lamps use sulphur, selenium, tellurium or compounds thereof. Additionally, metals and or/or metal halogens are added in small portions to the main fill substance to vary certain spectral characteristics of the omitted optical radiation. Although this type of lamps do not need to contain mercury for operation, the latter element is nevertheless added in some cases, sometimes even in large quantities, to aid in starting a discharge and to upgrade the lamp performance characteristics.
These lamps do not have the drawbacks inherent in the mercury containing electrode lamps. Their merit, as opposed to electrodes-based illumination lamps, is long service life, absence of heat loss since, there are no electrodes, a fairly smooth spectrum, and a relatively high power of radiation.
The closest analog of both the first and the second variants of the developed metal halogen electrodeless illumination lamp in terms of basic design similarities is a metal halogen electrodeless illumination lamp as disclosed in U.S. Pat. No. 5,864,210. Like the above described electrodeless gas discharge lamps, the nearest analog comprises a microwave generator coupled via a coupling means with a microwave cavity containing a discharge bulb. The discharge bulb fill comprises either one metal halogen from the group of indium, gallium and thallium halides, or a mixture of these compounds, the metal halogen containing either one element from the group of iodine, bromine and chlorine, or a mixture of these substances. By the action of a high frequency discharge initiated in the discharge bulb fill substance, the latter emits optical radiation having a molecular spectrum.
The discharge bulb also contains a small amount of an inert gas to aid in starting a discharge. Some part of the microwave cavity serves as a microwave screen which is constructed as a metal mesh that is opaque to microwave radiation while being practically transparent to optical radiation. To upgrade the luminous efficiency of the lamp, zinc is added to the discharge bulb fill, which increases the internal pressure in the bulb.
However, the above described lamps contains mercury, the toxic substance. Also, since there is a considerable loss of heat in these lamps to electrodes contained inside a quart tube, the lamp efficiency is degraded. In addition, vaporization of the electrodes darkens the quartz tube, resulting in that its brightness is weakened as time passes.
Therefore, an object of the present invention is to provide an improved metal halogen electrodeless illumination lamp that emits optical radiation having a molecular spectrum, and directed to develop a metal halogen electrodeless illumination lamp having enhanced performance characteristics compared to that of the conventional one.
To achieve these and other advantages and in accordance with the purposed of the present invention, as embodied and broadly described herein, there is provided a metal halogen electrodeless illumination lamp including a microwave generator coupled via a coupling means with a microwave cavity containing a discharge bulb and a microwave screen its function being performed by some part of the microwave cavity walls, the part being transparent to optical radiation. The discharge bulb includes a mixture of metal halogens which emits visible optical radiation characterized by a molecular spectrum, immediately when excited with a high frequency discharge occurring therein.